The ever-increasing demand for broadband communication systems has led to optical-transmission systems based on optical waveguides such as optical fibers and on optical processing elements for use in these systems. Generally, in high-performance communication systems, photons continue to supplant electrons as messengers.
Significant effort has been spent towards optical integrated circuits having high complexity and advanced functionality. As is described in Driessen et al., Proc. of SPIE Vol. 5956, 2005, which is hereby incorporated by reference herein, optical microresonators can be considered as promising building blocks for filtering, amplification, switching, and sensing. Active functions can be obtained by monolithic integration or by a hybrid approach using materials with thermo-optic, electro-optic, and optoelectronic properties and materials with optical gain.
In a common configuration in microresonator-based sensors, a microresonator is placed in close proximity to an optical waveguide such as an optical fiber whose geometry has been specifically tailored—for example, tapered or etched to a size of 1-5 microns. The tapering modifications to the waveguide result in there being a substantial optical field outside the waveguide, so that light can couple into the microresonator and excite its eigenmodes. These eigenmodes may be of various types, depending upon the resonant cavity geometry.
Optical microdisks or microring waveguides have been used in the past as both resonators and switches. Djordjev et al. describe a microdisk that can work as a tunable filter as well as a path switch for light of a particular wavelength (IEEE Photonics Technology Letters, vol. 14, no. 6, June 2002, pp. 828-830). The switching occurs when the resonance frequency of the microdisk resonator matches the frequency of the light in an input waveguide, thereby coupling light from that waveguide through the resonator into an output waveguide. When the resonance frequency is tuned to be different than the frequency of the waveguided light, that light is not coupled into the resonator but rather remains in the input waveguide. This switching behavior occurs for light whose wavelength or frequency is within a very narrow range of values.
An optical ring resonator whose switching behavior is controlled solely by controlling the coupling that occurs at its two resonator-to-waveguide coupling regions is described by Yariv (Electronics Letters, vol. 36, no. 4, February 2000, pp. 321-322). According to Yariv, the amount of light that remains in a first waveguide or that is coupled via the resonator into a second waveguide is controlled by adjusting the coupling coefficients of the two coupling regions—between first waveguide and resonator, and between second waveguide and resonator.
When the resonator is operated near its “critical coupling” point, the attenuation of the light in the resonator after a round trip is approximately equal to the amount of light coupled at the coupling region. At this critical coupling point, there can be perfect destructive (phase) interference at the output waveguide segment of a first waveguide between the light transmitted from the input waveguide segment of that first waveguide and the light coupled from the resonator into that output waveguide segment. When this perfect destructive interference occurs, all of the light is coupled into the resonator. That light, coupled into the resonator from the first waveguide, can be coupled almost completely out of the resonator through a second waveguide.
One type of filter known in the art is a cascade of 2×2 (two inputs and two outputs) finite impulse response (FIR) filtering stages such as described by K. Takiguchi et al., Journal of Lightwave Technology, vol. 13, no. 1, Jan. 1995, pp. 73-82. Each FIR stage contains an optical waveguide 2×2 coupler that divides the light into a pair of waveguide paths and then another optical waveguide 2×2 coupler that combines the light from those two paths. Thus, each FIR stage can resemble a Mach-Zehnder interferometer, having, in general, two arms of unequal lengths.
Another type of filter is a cascade of 2×2 infinite impulse response (IIR) filtering stages such as described by K. Jinguji, Journal of Lightwave Technology, vol. 14, no. 8, Aug. 1996, pp. 1882-1898. This filtering stage is similar to the FIR stage but has an optical ring resonator coupled to one of the waveguide arms. There can be an optical phase shifter in the other waveguide arm.
Examples of delay-line filter designs that contain both ring resonators and phase shifters are described by K. Jinguji and M. Oguma, Journal of Lightwave Technology, vol. 18, no. 2, Feb. 2000, pp. 252-259 and by C. Madsen, Journal of Lightwave Technology, vol. 18, no. 6, Jun. 2000, pp. 860-868. Both the optical ring resonator and the optical phase shifter can be located in the same arm of a Mach-Zehnder interferometer.
Optoelectronic devices that are based on a combination of dielectric optical waveguides (such as silica waveguides) and thin portions of semiconductor optoelectronic materials (such as InP or GaAs) are described in U.S. Patent No. 6,852,556 (D. Yap) and U.S. Patent No. 6,872,985 (D. Yap), both of which are hereby incorporated by reference herein. Devices described by these patents comprise at least one dielectric optical waveguide and a layer of “active” semiconductor material physically bonded to the dielectric waveguide material, wherein the “active” semiconductor material is able to generate light; detect light; amplify light; or modulate the intensity, phase, or polarization of the light.
In practice, both for convenience and for economic reasons, it would be beneficial if a simple unit cell could be suitable for the construction of filter building blocks comprising combinations of multiple unit cells. A preferred elemental unit cell would be versatile and could be used to construct the types of filters described by Jinguji et al. and Madsen, cited above, and other signal processors of various complexities and functionalities.
In view of the state in the art, there is a need for the aforementioned simple unit cell, along with methods to make and use such a unit cell in a signal processor or filter. Further, there is a need in the art for a programmable optical microdisk capable of operating as a switch, as a coupler, and as a resonator element in a delay-line filter. Finally, there is a need to integrate an optical microdisk with an optical phase shifter both coupled to the same optical waveguide.